Joe Ok lets talk about case strength! Then why is the wsm made with a big base or rim, to absorb pressure I would say. so why try and make it smaller, someone that designed the case must have thought it was a good Idea.
Joe - The WSM is made with the rim size it has so it will fit a magnum bolt face. It too is a rebated case. The 284 is rebated so it will fit a 308 bolt face. If the 284 had the same size rim as it's base it wouldn't fit in any guns.
To understand the function of the head on a case you will need a minimum understanding of the different strengths of materials. Two properties of great importance here are compressive strength and tensile strength. Lets compare it to something everyone can relate with.... Bubble gum!
Compressive strength: Picture a piece of bubble gum on a counter top. If you pressed your thumb on it, it will compress and squish out the sides. This would be similar to the chamber pressure squishing the case head against the bolt face.
Tensile strength: This would be how much the material will resist being pulled apart. Take a sticky piece of bubble gum, stick it to something light and pick it up. The gum doesn't stretch out because you have not exceeded its tensile strength. Stick it to something heavy and it will stretch. The tensile strength has been exceeded. This is what we are most worried about when there is unsupported brass in a bolt/chamber interface.
The inside diameter of the case is your thumb on the gum. The chamber PSI is applied to the surface area on the of the inside of the head. If there is solid material between the inside diameter of the case and your bolt head, compressive strength is what keeps your case head from swelling. This is why people measure the head and groove diameters when working up loads. It is a very good indicator of chamber pressure.
If you rebate a rim so there is a lack of barrel or bolt supporting the brass, only the tensile strength of the brass will contain the chamber pressure. The thinner the brass gets, the less pressure it will be able to contain. The WSM and 284 cases are not rebated enough to compromise the head strength to where tensile strength is more of a limiting factor than compressive strength. A WSM case rebated to 308 head size I believe would.
This would be a good place for another analogy. Imagine your gum dried out and got harder. Both strengths have increased. This is what happens when brass is hardened as well.
